In general, thermal spraying techniques and apparatus for coating metallic substrates are well known in the art. Thermal spray processes normally include the generation of a jet of high temperature carrier medium and the injection of a coating material into the carrier medium. The coating materials are usually powders which become heat-softened or melted by the carrier medium and propelled thereby at high velocities against the surface of a substrate to be coated.
Plasma flame spray apparatus typically include a plasma generator coupled to a nozzle. A plasma stream generated by the plasma generator is passed through the nozzle and towards a workpiece. A coating material is injected into the nozzle and the plasma stream and heat-softened or melted by the plasma stream and propelled thereby towards a workpiece applying the coating material thereto.
For example, U.S. Pat. No. 4,256,779 to Sokol et al. (hereinafter referred to as “Sokol”) discloses a plasma spray method and apparatus which is known in the industry as the Gator-Gard System manufactured by Sermatech International, Inc. of Limerick, Pennsylvania. Generally, the Gator-Gard System includes a plasma generator coupled with a nozzle for providing a plasma stream having improved characteristics. Upon entering the nozzle, a plasma stream is passed through a plasma cooling zone defined by a plasma cooling passageway, to a plasma accelerating zone defined by a narrowed passageway that expands into a plasma/particle confining zone. The narrowed passageway is cooled and the powder material to be applied to a substrate is introduced into the plasma stream along the cooled narrow passageway. Thereafter the particles and the plasma stream are discharged from the apparatus.
Similarly, in U.S. Pat. No. 5,858,469 to Sahoo et al. (hereinafter “Sahoo”), a modified thermal spray apparatus and method is disclosed. Generally, Sahoo discloses that plasma spray coatings of increased hardness can be applied to a workpiece by extending the distance at which the apparatus can spray the plasma/particle stream towards the workpiece. In the Sahoo apparatus, this is achieved by lengthening the passageway which defines the plasma/particle confining zone of the nozzle of the thermal spray apparatus. Additionally, Sahoo discloses that improved coatings can be obtained by increasing the ratio of the length to the diameter of the passageway defining the confinement zone.
One known problem associated with the plasma/particle confinement zones present in the nozzles of both of the Sokol and Sahoo apparatuses is that the particles of coating materials passing through these confinement zones can become cooled too much prior to exiting the nozzle. This can cause the particles of coating material to become clumped together resulting in imperfections in the applied coatings.
Further, the confinement zone present in the above-identified prior art nozzles necessitates a nozzle portion downstream of the material feed tube. This increases the minimum distance between the outlet of the nozzle and a workpiece during use of the nozzle in a coating process.
Another disadvantage of the above-identified prior art nozzle assemblies is that the nozzle assemblies do not include means for cooling the workpiece prior to applying a coating material.
Further, the above-identified prior art nozzles do not include means for enveloping the plasma stream exiting the nozzle with pressurized air for reducing contamination of the plasma stream discharged from the nozzle.
Based on the foregoing, it is the general object of the present invention to provide a nozzle assembly for use with known thermal spray apparatus that improves upon, or overcomes the problems and drawbacks associated with prior art nozzles.